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Old Nov 09, 2014, 08:43 AM
Registered User
Zurich
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Question
why use leading edge extensions on small models?

I can understand why fullsize aircraft sometimes use L.E. extensions on the outer or mid-span wing sections, but why use them on small models in which Re is critical for performance? My own designs -- which usually fly well above-average throughout a very broad flight regime -- employ wider chord inner sections to better raise Re and thus overall L/D. And in the case of unreinforced foam wings, add structural strength where it is most needed under flight load. But I continually and increasingly see >

http://ep.yimg.com/ay/yhst-172102528...ingspan-16.jpg

Why
do this? Or add outer-wing T.E. extensions?

What sense does it make for small RC planes?


curious/Lee



ref: http://en.wikipedia.org/wiki/Leading-edge_extension
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Old Nov 09, 2014, 10:29 AM
Jim C Patrick
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Originally Posted by xlcrlee View Post
. . . . Why do this? Or add outer-wing T.E. extensions? . . .
Because it looks more [cool, hot, great, wicked, insert your favorite superlative here] than another model without these features. The 'rocket science' is in the electronics, and there is no loss or gain from the minor resizing of the wingplan.
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Old Nov 09, 2014, 11:10 AM
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Originally Posted by jcpatrick View Post
Because it looks more [cool, hot, great, wicked, insert your favorite superlative here] than another model without these features. The 'rocket science' is in the electronics, and there is no loss or gain from the minor resizing of the wingplan.
I agree, given the target market.

And that seems quite reasonable from a mktg P.O.V.

In my silly brain, I rather first pay attn to the technical points I noted in the OP, plus due to far too many Charlie Brown experiences with kite-eating trees, I see just one more thing to get stuck in high branches. But that also helps mktg, as often a tree-bound plane must get replaced.

L
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Old Nov 09, 2014, 11:20 AM
internet gadfly
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It's not about Reynolds number or surface area. It's about the vortex that is generated by the sharp discontinuity at the step in the leading edge at high angle of attack. This vortex causes the section of wing outboard of the step to act like it is a low aspect ratio wing with the late stall i.e. low AR wings stall later than high AR ones. It's identical to the effect of a vortilon. A fence also has a similar effect but whereas the cuff and vortilon don't start producing a vortex until the wing has a fairly high AoA the fence is always "on" so to speak and therefore producing both parasite and induced drag.


---------.~.
--------/V\
------//----\\
-----/(------)\
----(^^)---(^^)--Norm
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Old Nov 09, 2014, 11:55 AM
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Zurich
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Originally Posted by nmasters View Post
It's not about Reynolds number or surface area. It's about the vortex that is generated by the sharp discontinuity at the step in the leading edge at high angle of attack. This vortex causes the section of wing outboard of the step to act like it is a low aspect ratio wing with the late stall i.e. low AR wings stall later than high AR ones. It's identical to the effect of a vortilon. A fence also has a similar effect but whereas the cuff and vortilon don't start producing a vortex until the wing has a fairly high AoA the fence is always "on" so to speak and therefore producing both parasite and induced drag.


---------.~.
--------/V\
------//----\\
-----/(------)\
----(^^)---(^^)--Norm
Norm, actually I totally understood & understand that. Pardon any ambiguity, but my question was specifically about its use in small models. That is, my experience shows me that Re is a far more important factor at very low Re. I mean, the L/D "goes off a cliff".

Can you elaborate or extend this relevance as it applies to these small models? At which chord-size/airspeed is there a crossover, if any?

Thanks!

Lee


EDIT: there are other ways of preventing or delaying tipstall, esp. w/outboard ailerons and modern "giro"-assisted control [programming].
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Old Nov 09, 2014, 03:11 PM
internet gadfly
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Vortex formation is not dependent on the Reynolds number. Mosquitoes generate little vortices and the great red spot on Jupiter is a vortex. Andy Lennon devoted a few paragraphs to the "NASA safe wing" in an article in Model Airplane News that is reproduced in a book that is still available for about $25. I have acumulated some refferences regarding leading edge devices, including the NASA leading edge cuff, and posted the ones that are relavent to this thread here. A leading edge device, such as a NASA cuff or a vortilon, is much more effective than ailerons because the cause of tip stall is simply that the outboard portion of the wing reaches its CLmax before the inboard part. By reflexing the ailerons you decrease the camber of that part of the wing and therefore decrease its CLmax. These leading edge devices increase the CLmax by delaying separation to several degrees higher than the nominal airfoil. NASA did the configuration work for the leading edge cuff with an RC model before they went on to a more expensive full scale airplane. This video shows some of the model flights. Sorry if it doesn't play in this message window, sometimes they (government bureaucrats) block imbedded videos so you might have to go to Youtube to watch it:
Segmented Wing Leading-Edge Devices (18 min 19 sec)
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Old Nov 09, 2014, 05:55 PM
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Thanks, Norm! From the video I am convinced that the segmentation was effective on that high-wing model. However, that model also seems larger than the ones to which I was referring. Further, I am reasonably sure that model would not have been chosen if it did not show the effect, else why publish the video?

How small or slow would a model have to be for the cuff-improvement to be insignificant?

How does it compare with "old-school" automatic or fixed slats?

http://109lair.hobbyvista.com/techre...lats/slats.htm

or drooped tips > http://www.homebuiltairplanes.com/fo...ding-edge.html

or conical camber? [which I used in the F-22 toy above]

I don't see it being used in rubber-powered indoor endurance models, "giant" TOC aerobatic models nor in high-performance fullsize sailplanes. Are they just ignorant? ['tis well possible]

Lee
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Old Nov 10, 2014, 06:27 AM
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Further, maybe more complete, thought:

It was clear to me from the start that the purpose of the widened outer wing chord [not denying the above mentioned "cool-looking" factor] is to delay or prevent tip-stall at slow speed and high AoA [not preventing which is however seemingly often necessary for aerobatic planes if they want to do good snap-rolls, etc.]. And most folks have the idea that washout is used to prevent such tip-stall. The problem with fixed washout is that it is only efficient in one relatively narrow set of conditions, which is why most modern aircraft, incl. jet fighters and airliners as well as large model + fullsize high-performance sailplanes [where in all cases efficiency is very paramount] use segmented T.E. flaps + ailerons to effectively VARY the washout for the various flight regimes/conditions.

Decades ago, while making a presentation at Grumman, the then-Chief of Advanced Design told me, irrelevant to the subject of that presentation [involved rotorcraft], that the majority of even aerospace engineers "believed" that washout was used to prevent tip-stall. He explained to me that it was not!

He said that at the fuselage and wing root the airflow was generally displaced and redirected downward w.r.t. the aircraft's flight path, that at some [unspecified] distance outboard of the tips the undisturbed flow was parallel to the flight path, and that this flow made a fluidic transition between, changing relative onflow direction along the span of the wing. Further, he said that every design has a different "pattern" of this transition from down to parallel [the fwd-swept wings of the plane they were then developing used wash-in, for ex. > http://en.wikipedia.org/wiki/Grumman_X-29, which their large RC spin-test contracted model-builder, who was also building rotorcraft models for me, separately confirmed].

The Chief of Advanced Design told me the purpose of washout [variable or not] was to get every section of the wing to be at an optimally efficient AoA.


Period.

And in my toy F-22 above, [which was designed & molded in an approximation of the wing's landing configuration for hopefully obvious reasons!], as in the fullsize aircraft, the conical camber "washout" is extremely effective. UN-cuffed, with no teeth needed [re: dogtooth > http://en.wikipedia.org/wiki/Dog-tooth].


Lee
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Old Nov 10, 2014, 07:51 AM
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Adding slats only on the outer portion of the wing will also alter its rigging angle slightly, effectively adding washout. Essentially, the unaltered, less cambered central section of the wing, will start to stall earlier and promote a more benign stall from the centre progressing outwards, leaving the ailerons in the un-stalled portion of the wing for a longer time. Lee, what you say about washout on full sized planes might well be true. Especially for military planes, efficiency and performance trumps safety. But in general purpose planes and models the washout is there to optimize lift distribution AND to provide for more benign handling near the stall.
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Old Nov 10, 2014, 08:25 AM
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Quote:
Originally Posted by xlcrlee View Post
The Chief of Advanced Design told me the purpose of washout [variable or not] was to get every section of the wing to be at an optimally efficient AoA
Quote:
Originally Posted by Brandano View Post
Especially for military planes, efficiency and performance trumps safety. But in general purpose planes and models the washout is there to optimize lift distribution AND to provide for more benign handling near the stall.
Actually, what I wrote was incomplete: he meant that the benefit of having good stall characteristics was a happy byproduct of the washout. Even though one follows the other, I suppose it is useful for designers, engineers, inventors & theoreticians to be aware of his point/s. For ex., I have seen some RC toy planes which use small outboard props for roll control. Presumably an "AS3X"-type system could use them, the same way the tip-jets on the fullsize Harrier do in hover, to stabilize, even in an otherwise stalled condition. I hardly think such an automatic steering-prop stabilized system would be considered efficient .... but in regard to a general aviation aircraft [OK, as with ALL aircraft] neither is crashing "efficient".

L
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Old Nov 10, 2014, 08:47 AM
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Zurich
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demo of what I meant by the conical-camber "washout" in the toy F-22 with no movable control surfaces being "efficient" [note that the slight rocking was in aerodynamic stabilizing response to my imprecise "gyroless" crude motor-control inputs to hit the landing spot!] >


Indoor Aircraft Carrier Landing (0 min 5 sec)





here it is going faster outside in wind >


Micro Fighter /StarScream RTF R/C F-22 ORIGINAL version (0 min 42 sec)
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